The present invention relates to a combustion chamber construction for an engine which is applied to a combustion chamber in a cylinder head of a gasoline engine or the like.
Inlet ports and exhaust ports are formed in a cylinder head of an engine such as a gasoline engine, and the ports are designed so as to be opened and closed by inlet valves and exhaust valves at predetermined timings. In addition, there is known an engine which includes a variable valve lift mechanism which can change opening degrees or lift amounts of inlet valves in accordance with the operating conditions of the engine.
In the engine including the variable valve lift mechanism, an improvement in exhaust emissions or an increase in fuel consumption is realized by decreasing the lift amounts of the inlet valves and producing a tumble flow of inlet air/fuel mixture in the combustion chambers when the engine load is small, for example. Because of this, it is an important subject to realize the generation of a stable tumble flow and a strong flow of inlet air/fuel mixture when the lift amounts are small. The tumble flow means among air or air/fuel mixture flows generated in the combustion chamber an air or air/fuel mixture flow which is a swirl having a spiraling center in a direction which intersects a reciprocating direction of a piston at right angles and which is also called a vertical swirl.
In order to generate a desirable tumble flow in a combustion chamber, for example, Patent Document 1 discloses a proposal in which a flow promoting portion called a shroud is formed at part of the combustion chamber in a side lying farther from an exhaust port, so as to promote the generation of a tumble flow. In addition, Patent Document 2 discloses a proposal in which the flow of inlet air/furl mixture is promoted by forming a guide portion made up of a recess portion on an opposite side to the shroud in a side lying nearer to the exhaust port.    [Patent Document 1] Japanese Patent Publication No. 2006-329131 A    [Patent Document 2] Japanese Patent Publication No. 2008-274788 A